When constructing a new building or the like, it is nearly always necessary to first excavate the ground upon which the building is to be erected so that the required footings can be constructed. The usual procedure is to excavate a suitable hole in the ground. The walls of this hole require support so that they will not collapse while the footings are constructed. This support is especially required when constructing a building very close to an existing building. In the absence of a suitable support, the foundations of the existing building may at least move or, at worse, collapse into the adjacent excavation.
For the deep excavations necessary for modern tall buildings such as office blocks and hotels, it is often a requirement that the ground be excavated to a depth of at least six meters. Further, to prevent any movement of adjacent buildings, it is often a requirement that the retaining wall(s) be constructed below ground level before any excavation is undertaken.
Similarly, when constructing a new dam, it is often necessary to provide some form of retaining wall around the dam site to prevent passage of the soil etc into the body of water as the dam fills and, also, to retain the body of water itself in the designated area.
Traditionally, such retaining walls have been constructed by the technique of "piling" wherein interlocking piles, each pile being approximately 0.33 m wide and up to 6 m long, are driven (by the repetitive application of a large impact force) into the ground to form a wall of the required width. This method is time consuming and expensive as a large number of piles have to be first driven into the ground and subsequently removed after the building foundations, walls etc have been constructed. Further, the bulk of the piles and the equipment necessary to first support and then drive them into the ground prevents a below-ground retaining wall from being built extremely close to an existing wall of a building or the like. In addition, the large force necessary (e.g. from an impact driver) to drive the piles into the ground results in shock waves through the ground which can damage the foundations of an adjacent existing building.
One alternative method that has been tried is to replace the afore-mentioned piles with sheets of metal. These sheets of metal, which are substantially wider than the piles, are also driven into the ground. However, although this alternative method reduces the time required to erect a retaining wall of given length, it is still necessary to remove the sheets after the building foundations, walls etc have been constructed as the sheets are too expensive to be used once only and thus left in the ground for all time. Further, the gauge of these sheets are such that, to position these sheets, it still requires the above mentioned traditional bulky equipment and the application of large impact forces with their attendant problems.
A modification of this known technique--described in UK Patent No 1074054 --is first to drive a preliminary ramming plate of relatively heavy gauge into the ground which is then withdrawn and a second plate--of lesser gauge--is then inserted into the "slot" thus created.
Although this modified method (using a lesser gauge and thus cheaper to manufacture sheet) may allow the second plate to remain in the ground for all time, it is essential always to create the "slot" before the actual second plate can be inserted into the ground. Thus the time to erect a retaining wall is not significantly reduced and the ramming of the first plate into the ground still involves large impact forces with their accompanying problems.
When constructing a revetment, it is nearly always necessary first to drain the water from where the revetment is to be erected so that the required footings can be constructed. Alternatively, where draining is not practical (e.g., on a foreshore), work on footings and the like can only be undertaken at low tide or by first diverting the flow of water away from the construction area.
The revetments are then traditionally constructed from rock or stone which is positioned where required, either dry-stacked or, if necessary, further held in position by cement or by placing a net-like structure (usually manufactured from metal) thereover. These methods are time consuming and expensive as they are labour intensive and, usually, the rock or stone has to be carted from an area remote from the revetment construction.
One alternative method that has been tried is to prepare the land where the revetment is required to provide the desired contour for the revetment, for example, by excavation or mounding with earth or rubble and placing thereover a double walled mattress which is then filled with concrete by pressure injection. This alternative method is still not totally satisfactory as, for example, if excavation is required to set the desired contour, draining or diverting of water may still be required.
Further problems with the known prior art include (1) for the deep excavations necessary for modern tall buildings, the required retaining walls often have an exposed face which is of substantial depth and thus the ground pressure on the other side can be substantial and may cause these retaining walls to move inward away from the required angle under the influence of this pressure; and (2) similarly, when such walls or revetments are placed in soft soils such as sand or similar, particularly when wet, movement of the walls is likely.
Traditionally, such retaining walls have been anchored by using a concrete grout wherein a threaded hole is bored into the soil, concrete is then poured into the hole and metal cables are embedded therein. Once the concrete has cured, the metal cables are secured to the retaining wall to prevent movement thereof.
Disadvantages of this traditional method include (1) soil has to be removed before the concrete is poured in, requiring special drilling equipment; (2) several days are required for the concrete to cure before the grout can be used; (3) if insufficient grout is added to replace the removed soil, subsidence can occur of the surrounding area; (4) the drilling equipment is bulky and problems thus arise if the grout has to be placed near existing foundations of, for example, an adjacent building; (5) the anchoring system has to be "destressed" in due course; (failure to destress is highly likely to allow subsidence and other movement of the surrounding soil which could cause damage to the new construction and/or to adjacent buildings); and (6) a concrete grout is permanent, there being no reusable materials.
The above discussion has described existing problems associated with the construction of support walls which are to be erected in an essentially vertical position. However, "horizontal" supports of considerable strength are also required, for example, in large suspended floor spans such as those necessary in modern office and retail complexes and, particularly, in the construction of roads, bridges and the like.
The traditional bridge building material, timber, is now out of favour as its cost is increasing and supplies are becoming more difficult to obtain. Timber bridges also require significant regular maintenance. Accordingly, the repair and replacement of the decking of existing wooden bridges and the construction of new bridges now tends to be undertaken using alternative materials, most commonly reinforced concrete or steel sheets covered by some suitable load bearing material.
Although reinforced concrete is immensely strong and durable, large and thus expensive quantities are required if the concrete is to be the only supporting surface. Extensive formwork is also required to contain the concrete until it has set. Therefore, in an attempt to overcome this problem, steel decking has been utilised whereby profiled steel panels are first laid down and then covered with any suitable infill material. These fill materials vary from compacted earth to structural grade concrete.
A disadvantage of this steel decking alternative is that the road surface is not load bearing until the infill material has been positioned. This usually necessitates the infill material to be positioned manually as the initial steel decking is not strong enough to support the large and heavy vehicles, such as concrete-containing vehicles, which deliver the material. The required manual distribution of the infill material is time consuming, labour intensive and thus relatively expensive.